In a relatively small-sized vehicle which requires a small-sized transmission assembly for propelling the vehicle, a transmission assembly is often employed in which a hydrostatic transmission which may be designed in a compact fashion, and a mechanical transmission mechanism, which is designed in a compact fashion, are connected in series. And, in a case where the mechanical transmission mechanism is disposed within a transmission casing which supports left and right rear wheel axles for driving the vehicle through left and right rear wheels, the hydrostatic transmission is often mounted on an outer surface of the transmission casing. Pump shaft of the hydraulic pump of such hydrostatic transmission is often used as a power source for a rear-PTO (power take-off) shaft which extends rearwardly from the transmission casing for driving an auxiliary implement, such as a grass collector, to be drawn by the vehicle. The pump shaft is often used further as a power source for a mid-PTO shaft which extends forwardly from the transmission casing for driving an auxiliary implement of mid-mount type, such as a mid-mount mower, to be mounted at an underside of the vehicle and between front and rear wheels.
Typical examples of the vehicle transmission assembly of this type are disclosed in JP, B2 No.1-17883; JP, A No.62210132; and JP, A No.1-175526.
In the transmission assembly disclosed in JP, B2 NO.1-17883,a hydrostatic transmission comprising a hydraulic pump and hydraulic motor which are disposed respectively at higher and lower levels is mounted on a front surface of a transmission casing. Within the transmission casing, there are disposed not only a mechanical transmission mechanism for transmitting vehicle-driving power but another mechanical transmission mechanism for transmitting auxiliary implement-driving power to a rear-PTO shaft, extending rearwardly from the transmission casing, and to a mid-PTO shaft which extends forwardly from the transmission casing at a level below the hydrostatic transmission. In correspondence to the pump and motor arrangement in the hydrostatic transmission that the hydraulic motor having a motor shaft from which vehicle-driving power is transmitted into the transmission casing is located below the hydraulic pump having a pump shaft from which auxiliary implement-driving power is transmitted into the transmission casing, the mechanical transmission mechanism for transmitting vehicle-driving power is disposed at a lower half within the transmission casing while the mechanical transmission mechanism for transmitting auxiliary implement-driving power is disposed at an upper half within the casing. The transmission mechanism for transmitting auxiliary implement-driving power includes a fluid-operated frictional PTO-clutch which is operable to connect selectively between a drive shaft, which is disposed at a high level within the transmission casing and is coupled to the pump shaft, and a clutch shaft which is disposed below the drive shaft and on which the PTO-clutch is mounted.
In the transmission assembly disclosed in this JP, B2 NO.1-17883, the mechanical transmission mechanism for transmitting vehicle-driving power within the transmission casing includes transmission shafts, which extend axially of the transmission casing, and a differential gearing which is disposed behind the transmission shafts and has, as its output shafts, left and right rear wheel axles. Consequently, the transmission casing is still pretty large in axial length. Further, the rear wheel axles are located at a rearmost portion of the transmission assembly having the hydrostatic transmission and transmission casing so that turnability with a small radius of turn is not assured particularly when the transmission assembly is located in a vehicle as rearwards as possible so as to secure a large axial space for a mid-mount mower or the like between front and rear wheels. With respect to a small-sized vehicle, turnability with a small radius of turn is very important.
The transmission assembly disclosed in JP, A No.62-210132 is intended to eliminate the problems set forth above and comprises a hydrostatic transmission which is located at a rearmost portion in the transmission assembly. The hydrostatic transmission comprises a hydraulic pump and hydraulic motor which are disposed respectively at higher and lower levels. To a lower half of front surface of the hydrostatic transmission, a transmission casing is attached at an upper half of the casing into which a motor shaft of the hydrostatic transmission extends. Within the transmission casing, a mechanical transmission mechanism for transmitting vehicle-driving power is disposed which includes plural transmission shafts all extending laterally of the transmission casing and which is connected drivenly to the motor shaft through a bevel gearing. Pump shaft of the hydraulic pump has an input end extending forwardly at a level above the transmission casing. This pump shaft also extends rearwardly so as to be used as a rear-PTO shaft.
In the transmission assembly according to this JP, A No.62-210132, axial length of the transmission casing is largely reduced owing to the lateral arrangement of all of the transmission shafts. Further, left and right rear wheel axles supported by the transmission casing which is arranged in front of the hydrostatic transmission are located at a foremost portion of the transmission assembly so that, even when the transmission assembly is located in a small-sized vehicle as rearwards as possible, turnability of the vehicle with a small radius of turn is well secured.
On the other hand, the level of the pump shaft which is used as a rear-PTO shaft is too high so that a transmission shaft for connecting the pump shaft to an input shaft of a grass collector or the like takes a large inclination resulting in generation of a large noise at universal joints at the ends of the transmission shaft. Another problem consists in how a PTO-clutch is provided. Further, the transmission assembly disclosed in this JP, A No.62-210132 does not permit to use the pump shaft as a power source for driving an auxiliary implement of mid-mount type such as a mid-mount mower. Indeed, in the mower tractor disclosed in this literature, it is fashioned that power for driving a mid-mount mower is taken off directly from an engine.
Finally, the transmission assembly disclosed in JP, A No.1-175526 is fashioned, similarly to the one disclosed in JP, B2 No.1-17883, such that a hydrostatic transmission comprising a hydraulic pump of a higher level and a hydraulic motor of a lower level is mounted on a front surface of a transmission casing within which a mechanical transmission mechanism for transmitting vehicle-driving power, having forwards and rearwards extending transmission shafts, is disposed at a lower half of the casing. In the transmission assembly disclosed in this JP, A No.1-175526, however, a drive shaft which is disposed at a high level within the transmission casing and is coupled to a pump shaft of the hydraulic pump extends rearwardly from the transmission casing. On a rear end portion of this drive shaft, a hollow clutch shaft extending through a rear end cover of the transmission casing is rotatably mounted which is connected selectively to the drive shaft by an electromagnetic PTO-clutch disposed on the rear of the transmission casing. The clutch shaft is connected drivingly to a rear-PTO shaft, extending rearwardly from the transmission casing, and to a mid-PTO shaft, extending forwardly from the transmission casing at a level below the hydrostatic transmission, through gear trains which are disposed at a rear end portion within the transmission casing.
As compared to a PTO-clutch disposed within a transmission casing, a PTO-clutch disposed at a location outside of a transmission casing such as the one set forth above will heighten the freedom of design of a transmission assembly and, also, will make the assembly procedure and maintenance of it easier. However, it requires to take measures for safety and against water and dust. A fluid-operated frictional clutch which may be engaged in a smooth manner with ease cannot be employed as a PTO-clutch to be disposed at an outside of a transmission casing, because a fluid-operated frictional clutch has a structure such that oil for lubricating frictional elements of the clutch may overflow out of its clutch housing.